Method of storing reflection coeffients in a vector quantizer for a speech coder to provide reduced storage requirements

Abstract

An input speech signal is encoded as one or more reflection coefficients. To reduce storage requirements, the reflection coefficients are scalar quantized by storing an N-bit code rather than the entire reflection coefficient. An exemplary value for N is 8. A table is provided having 2.sup.N reflection coefficient values. The N-bit code is used to look up reflection coefficient values from the table. To reduce spectral distortion due to scalar quantization, the reflection coefficient values in the table are non-linearly scaled.

Claims

1. A speech coding method comprising the steps of:

(a) constructing an excitation codebook of 2.sup.M codevectors using M basis vectors;

(b) receiving input speech;

(c) in response to the input speech, computing reflection coefficient values corresponding to speech parameters representative of the input speech;

(d) storing in a table 2.sup.N reflection coefficient values, each reflection coefficient value addressable with an N-bit code;

(e) processing codevectors to produce synthesized speech;

(f) selecting a codevector from the excitation codebook which minimizes an error criterion for the synthesized speech relative to the input speech, including

(f1) when reflection coefficient values are required for processing, providing corresponding N-bit codes to the table to look up the reflection coefficient values,

(f2) otherwise storing only the N-bit codes during processing, thereby minimizing storage requirement for the reflection coefficient values.

2. A method of storing reflection coefficient vectors in a vector quantizer for a speech coder in accordance with claim 1 wherein the reflection coefficient values are non-linearly scaled.

3. A method of storing reflection coefficient vectors in a vector quantizer for a speech coder in accordance with claim 1 wherein the reflection coefficient values are arcsine scaled between the values of -1 and +1.

4. A method of storing reflection coefficient vectors in a vector quantizer for a speech coder in accordance with claim 1 where N equals 8.

5. A speech coder comprising:

a codebook generator which generates an excitation codebook having 2.sup.M codevectors formed using M basis vectors;

input means for receiving an input speech signal and producing a data vector;

coding means coupled to the input means for generating reflection coefficients corresponding to speech parameters representative of the input speech signal, the coding means processing the codevectors to produce synthesized speech;

a vector quantizer for quantizing the reflection coefficients, the vector quantizer including a vector quantizer memory configured to store 2.sup.N reflection coefficient values, the vector quantizer memory having a N-bit input and an output, the vector quantizer memory providing one of the 2.sup.N reflection coefficient values at the output in response to an N-bit address received at the N-bit input; and

a codebook search controller coupled to the codebook generator which selects a codevector from the excitation codebook to minimize an error criterion between the synthesized speech and the data vector, the codebook search controller being coupled to the vector quantizer and providing a corresponding N-bit code to the vector quantizer to look up a reflection coefficient value for processing, the codebook search controller otherwise storing only the N-bit code to thereby minimize storage requirements.

6. A speech coder as recited in claim 5 wherein each reflection coefficient value is related to an associated N-bit address by an arcsine scaling function.